This invention relates to the production of pneumatic tires from noncellular polyurea-polyurethane elastomers by casting, or centrifugally casting, liquid reaction systems.
The production of pneumatic tires based on polyurea-polyurethane casting systems is known. U.S. Pat. No. 4,029,730, for example, describes the production of a carcass sidewall using alkali metal salt complexes of 4,4'-diamino-diphenylmethane as chain extenders. With polypropylene oxide polyols, however, for example (as used in accordance with the present invention) chain extenders such as these do not give sufficiently favorable properties for pneumatic tires (cf. Tables 1 and 5A, Example 18). Another disadvantage lies in the high heating temperature (130.degree. C.) required in the process according to the above-mentioned U.S. patent, whereas according to the present invention, it is possible to work with considerably lower mold and heating temperatures (50.degree. to 80.degree. C.).
U.S. Pat. No. 3,963,681 relates to the use of certain mixtures of polyether polyols for producing cast tires. These particular mixtures are distinguished by the fact that they contain at least one polyether, the molecular weight of which is greater than the "critical rheological molecular weight". Under this assumption, a particularly good, permanent flexural strength is attributed to the elastomers produced in accordance with the reference. However, it has been found that the use of long-chain polyol fractions basically leads to a marked increase in the plastic deformation of the tires in the high-speed test (see Table 4: Examples 7, 8 and 9; dyn. peripheral growth at end of test). Thus, the present invention differs essentially in the fact that the particular effect of minimal plastic deformation under dynamic stressing may only be obtained using polyols with molecular weights considerably below those of the reference.
German Offenlegungsschrift No. 2,633,457 (U.S. Pat. No. 4,096,101) describes the post-cross-linking of elastomers with reactive fillers for the purpose of improving their properties. Apart from other differences, this reaction step is avoided in the process according to the present invention. German Offenlegungsschrift No. 2,622,951 describes a process for producing elastic moldings on the one-shot principle. The high reactivity, however, typical of the one-shot processing of formulations containing aromatic diamines, prevents the production of commercially satisfactory tires by casting or centrifugal casting. In addition, the one-shot elastomers have excessive plastic deformability.
U.S. Pat. No. 3,428,610 discloses cross-linked polyurethanes prepared from prepolymers and aromatic diamines. Catalysts are not used and the advantages of the instant invention are not recognized.
Now, by virtue of the present invention, it is possible to produce pneumatic tires combining low plastic deformability with high dynamic strength by casting, or centrifugal casting, from polypropylene oxide polyols which are both inexpensive and readily obtainable. The formulations used in accordance with the present invention show medium reactivity which, on the one hand, enables large (tractor) tires to be produced and, on the other hand, also provides for in-mold times of only fifteen minutes or less, hardening taking place at mold and starting material temperatures of from 50.degree. to 80.degree. C.